Plexiglass portholes

[brtilley]

Hi,

I have a sailboat that I am preparing for offshore cruising. I currently have relatively thin plastic portholes with plastic frames...in a steel cabin. These portholes will not survive heavy seas, so I was going to make metal covers (removable, either steel or aluminum) as the cost of new metal portholes is much more then my budget will handle...but then I will get no light into the cabin.

So I was thinking that maybe plexiglass covers might work if I knew the best thickness for this application. I do not know the actual force exerted by wave action on something 8" in diameter, so I though I would ask people who actually know these things. I would very much appreciate any advice or comments as to if this might work. Thank you very much...

[mgb_phys]

Perspex/Plexiglass (PMMA=Poly(methyl methacrylate)) is very strong.
I would probably worry more about how they are going to attach to the hull and how strong that connection is ( and how watertight) than about a 1/2inch thick perspex disc breaking.

It also has very good enviromental resistance, UV light and salt water don;t effect it.

[brtilley]

Thanks for the reply...I am going to use the holes of the existing 8 bolts per porthole, but install longer ones to attach the covers when needed using wing nuts. As the cabin is steel I believe the support will be sufficiently strong. They do not need to be waterproof, just deflect the force of large waves. The existing portholes are waterproof and should be OK. Thanks again.

[mgb_phys]

If you can make a metal ring to go around the outside of the perspex disc it will spread the force of the bolts better and avoid cracking where the bolt tightens against the perspex. Alternatively big washers and some pliant rubber like material under each bolt will help